Such backrests are well-known, and they are usually connected to the rear side of the seat by means of a fixed pivot. Frequently, they consist of a basic frame, with respect to which part of the support can be moved to the front in the lumbar region so as to follow the individual curvature of the spinal column in the lumbar region (indicated by letters “AB” in FIG. 1). Moving said support part to the front can be done mechanically or pneumatically. Frequently, said supporting region can be adjusted in vertical direction as well, since the location of the deepest point of the lumbar region of the spine can vary with each individual person. One drawback of these solutions is that only a marginal individual optimization can be achieved. The adjusting range for moving the lumbar support to the front with respect to the seat is limited.
U.S. Pat. No. 3,121,592 relates to a chair intended to provide more comfortable support for the lumbar region of the occupant's back. To this end, the chair according to this publication comprises a fixed shoulder engaging panel and an automatically self-adjusting lumbar engaging member.
French patent application 1.303.089, concerns a similar chair, wherein the adjustable support is positioned lower, i.e. more towards the seat.
Shortcomings of the prior art will be explained by means of FIG. 2 attached to this description, which shows several possible forms of the lumbar region of the spine, ranging from a concave back or lordosis (numerals 1 and 2), via a flat back (3) to a convex back or kyfosis (4), whilst the posture of the upper body remains unchanged. The position on the seat relative to the backrest shifts forward accordingly. FIG. 3 shows the same postures for the concave back and the flat back, whilst the position of the seat remains unchanged. The difference is remarkable. It is impossible to bridge this difference with existing specific lumbar supports. Moreover, the posture is influenced to a significant degree by moving the lumbar support to the front, whilst the desired posture has already been determined earlier by adjusting the angle of the backrest. From a functional point of view, this is not logical. Furthermore, the extent of support in transverse direction decreases when the lumbar support is moved to the front. As a result, the desired lateral support of the pelvis decreases. This also is not logical from a functional point of view.
The basic frame of existing backrests usually consists of a hard shell which is lined with a foam, or of a frame in which foam-covered non-sag springs or webs are stretched in transverse direction. Such constructions exhibit a limited capability of adapting to the individual shape of the back and the pelvis. Usually the thoracic-cervical transition region (indicated by the letter “C” in FIG. 1) is not supported, and in those cases where it is attempted to do so by forming the upper part of the backrest with a curvature to the front, said curvature is not individually adjustable.
Some important drawbacks of existing forms of individually adjustable posture support are that, from a functional viewpoint, the adjusting process is not based on a logical order, that the adjusting ranges are too small, and that there is no adjustable support for the upper part of the back. As a result, an optimum result is usually not achieved.